The present invention relates to a heavy duty tire, more particularly to a heavy duty tire having a bead structure capable of reducing the tire weight without deteriorating the bead durability.
A carcass ply which forms the skeleton of a pneumatic tire is usually anchored by turning up the both end portions of the ply around a pair of opposing bead cores. In recent years, a so-called bead wind structure is proposed as disclosed, for instance, in JP-A-2005-170145, wherein as shown in FIG. 5 each of turnup portions a1 of a carcass ply “a” is wound around a bead core “b” approximately one round. Since the turnup portion a1 terminates in the vicinity of the bead core “b”, a stress applied when the bead is deformed scarcely acts on the terminal end a1e of the turnup portion a1 and, therefore, the bead wind structure can effectively control a damage occurring from the terminal end a1e of the carcass ply, such as cord loosening. This structure also has the advantage of weight reduction, since the length of the turnup portion a1 is small. However, for tires having a bead wind structure, too, further weight reduction has been strongly demanded, for instance, by decreasing the rubber volume of the bead portions of tires.
On the other hand, tires having the bead wind structure show a relatively large bead deformation when contacting the ground as compared with tires having a non-bead-wind structure. In order to prevent the main portion a2 of the carcass ply “a” from falling down in the axially outward direction, it is known to reinforce the bead portion by disposing a bead reinforcing layer “c” having a U-shaped cross section in the bead portion. The thus disposed bead reinforcing layer is composed of a curved portion extending radially inward of the carcass turnup portion, and axially inner and outer portions extending radially outwardly from the curved portion. However, a shearing force greatly acts on the radially outer end c1e of the axially outer portion c1 of the bead reinforcing layer to damage the outer end c1e. The shearing force can be decreased by increasing the rubber volume of the bead portion at the outer end c1e to decrease the strain, but this results in increase in tire weight.
It is an object of the present invention to provide a heavy duty tire which can achieve weight reduction of the tire by reducing the weight of the bead portion while securing the bead durability.
This and other objects of the present invention will become apparent from the description hereinafter.